gdb: Remove stabs support for COFF files

gdb: Remove stabs support for COFF files

This commit continues the removal of stabs by removing support from coff
inferiors. This is trivial for the most part, just a removal of code
setting things only relevant for stabs. A couple of things could do with
some explanations:

The global variables symnum and within_function were kept (and
within_function was converted to boolean). I looked into making them
parameters to the relevant function, but this would require changes to
several otherwise untouched functions, so I kept them as globals
instead.

Reading coff debuginfo also uses a symbol chain similar to stabsread's
global_sym_chain. WIP: NEED TO FIX THIS PART.